Many discerning audiophiles and musicians are demanding ‘high resolution’ digital audio, which is normally understood to a mean audio sampled at a frequency significantly higher than the 44.1 kHz or 48 kHz of current media and quantised with a resolution better than 16 bits.
Lossily compressed audio is commonplace in the consumer market, but experience has led many people to be suspicious of lossily compressed audio, even of systems that claim to be ‘transparent’. An exception is plain nonadaptive noise-shaped dithered requantisation to a constant bit depth. With proper precautions this is equivalent (according to first-order and second-order statistics of the difference between input and output) to the addition of a constant noise (see J. Vanderkooy and S. P. Lipshitz, “Digital Dither: Signal Processing with Resolution Far below the Least Significant Bit” in Proc. AES 7th Int. Conf. on Audio in Digital Times (Toronto, Ont., Canada, 1989), pp. 87-96), which is considered ‘benign’ as a result of decades of experience with both analogue and digital media.
Two music distribution media dominate the mass market: the compact disc (CD) which has a sampling frequency of 44.1 kHz and a bit-depth of 16 bits, and the Internet download typically heard through a computer or personal player. Although most downloads are lossy-compressed, the computers or players are almost invariably able to handle uncompressed PCM (Pulse Code Modulation) signals at sampling frequencies of 44.1 kHz and 48 kHz. Many can handle bit depths of 24 bits, though some personal players are restricted to 16 bits.
It is commercially unattractive to issue audio recordings in both an audiophile version (having a sampling frequency of typically 96 kHz) and in a format that can be played on mass-market players. The possibility of issuing a recording that is playable on standard mass-market players but also contains hidden information that allows a special decoder to retrieve additional bandwidth has been explored several times previously, including by Komamura MITSUYA KOMAMURA “Wide-Band and Wide-Dynamic-Range Recording and Reproduction of Digital Audio” J. Audio Eng. soc. Vol. 43, No. 1/2, 1995 January/February). However none has so far provided standard PCM playback compatibility while addressing the desire for lossless retrieval of an original higher-sampling-rate signal and none has considered the question of how a decoder may provide an optimal experience to the listener at two different bit depths (for example for both 16-bit and 24-bit players).